Vacuum can 1965 327/350hp

[John P.]

Hello folks,

At idle the temp on my roadster is climbing to 220. I know there are several things to evaluate, timing and total advance being among them.
I have reviewed Duke's multiple posts on vacuum cans and full manifold vacuum. My question is which can for my particular engine? To my knowledge it is stock but Service Manual specs suggest a "special" cam for this engine.
Currently I have a B-28 installed which appears to pull full advance at 8mmHg when testing with MityVac-if I am understanding its function correctly.
Thoughts on the recommended can on this particular engine? Thanks in advance.

[Gene M.]

Re: Vacuum can 1965 327/350hp

What do you mean by "Service Manual specs suggest a "special" cam for this engine. "? Was the cam changed to something other than stock equipment?

Is the entire cooling system up to spec? Radiator, fan and clutch, thermostat, hoses, etc?

[Tony S.]

Re: Vacuum can 1965 327/350hp

John, how old is your radiator? Is it the original? I had a similar overheating problem with my original March 1965 radiator. When I replaced it with a new DeWitt, it stays pretty consistently at 180 degrees.

[Duke W.]

Re: Vacuum can 1965 327/350hp

The L-79 did use a cam that was unique to that engine configuration, so I guess you can call it "special". With the OE or OE equivalent cam the L-79 should pull 14-15" manifold vacuum at about 750 in Neutral, so you should check that by teeing your Mity Vac into the VAC signal line, and total idle advance with the VAC connected should be in the mid twenty degrees range.

The OE 236 16 and current B28 VACs are more aggressive than necessary for the L-79, and I recommend a 12" B26 if it needs replacement. Search for a thread stated by me in December 2012 - L-79 distributor blueprint overhaul. We went with the less aggressive B26, but since the OE centrifugal curve is very lazy, we used lighter springs to bring it all in sooner and also assembled the distributor with much tighter tolerances than the typical sloppy OE assembly. Both I and the owner were very satisfied with the engine's broad torque bandwidth performance.

It sounds like your B28 is functioning properly, and if so, the spark advance map is not likely the source of your high operating temperatures, so you need to check other issues like the fan clutch, radiator configuration and age, I'm amazed at how many sixties vintage Corvettes still have the original Harrison radiator, but they are all living on borrowed time. If you have an OE Harrison or a aftermarket brass replacement radiator, you should definitely consider installing the DeWitts exact reproduction of the original Harrison.

A weak fan clutch can cause high operating temperatures in low speed stop-and-go driving, but not at sustained speeds above about 35 MPH. A clogged radiator may be the problem if it runs hot all the time.

You should also check surface temps from the engine outlet to the radiator inlet with an IR gun as old temperature sensors can "drift" with age and some replacements are not calibrated correctly.

Duke

[John P.]

Re: Vacuum can 1965 327/350hp

Has a Harrison repro installed about 6 years ago. Seems that it did not have this problem initially.

In the 1965 supplement to the 1963 service manual all engines are listed in the specs section in the back The 327/350 combo lists a Holley and a special cam. No other info.

[John P.]

Re: Vacuum can 1965 327/350hp

I suspect fan clutch is original and shrouds appear intact. Is Dewitt the only maker of Harrison replacement radiator? It has an original appearing Harrison sticker.
I plan to obtain an IR temp gun to confirm that I really have an issue.

[Duke W.]

Re: Vacuum can 1965 327/350hp

Chevrolet liked to use the word "special" to refer to high performance camshafts. Back in the fifties the Duntov cam was often referred to in marketing and service literature as "special camshaft".

Duke

[Duke W.]

Re: Vacuum can 1965 327/350hp

I suspect fan clutch is original and shrouds appear intact. Is Dewitt the only maker of Harrison replacement radiator? It has an original appearing Harrison sticker.
I plan to obtain an IR temp gun to confirm that I really have an issue.

Easy fan test: With the engine cold turn the fan by hand and note the effort. Then let it get up to over 200 at idle, shut down the engine and turn the fan again. It should require much more effort. Turn it at least one full revolution in both tests. Effort should be consistent through one full revolution with no soft spots.

Most replacement clutches are "tuned" for later emission controlled engines with 195 degree thermostats, so they won't tighten until higher operating/radiator exit air temperature, which can still result in higher than normal around town driving temperatures.

The original can be rebuilt by specialists. I can't recall the names right now, but they've been mentioned many times on the TDB.

Another test: before the engine is fully warmed up free rev it to about 3000, then again with the temperature at 200 or above and you should hear more "fan roar" and feel much greater air flow coming off the fan if it is in proper working order.

When cold the fan won't go much above about 1500 regardless of engine revs. With the clutch fully tightened it will rev to about 3500.

Duke

[Joe L.]

Re: Vacuum can 1965 327/350hp

I suspect fan clutch is original and shrouds appear intact. Is Dewitt the only maker of Harrison replacement radiator? It has an original appearing Harrison sticker.
I plan to obtain an IR temp gun to confirm that I really have an issue.

John------


Other than Harrison, DeWitts is the only manufacturer of the stacked plate design aluminum radiator. However, there are copper/brass replacements out there and sometimes folks put a Harrison foil label on these. The copper/brass replacements will not cool as well as the stacked plate aluminum radiators, especially in challenging cooling environments.